Photoelectrophoretic imaging process

ABSTRACT

A photoelectrophoretic imaging system in which selected wavelengths of electromagnetic radiation are removed from the imagewise exposing illumination.

United States Patent Carreira et al.

[ 1 May 6,1975

PHOTOELECTROPHORETIC IMAGING PROCESS Inventors: Leonard M. Carreira, Penfield;

Vsevolod Tulagin, Rochester, both of NY.

Assignee: Xerox Corporation, Stamford,

Conn.

Filed: Aug. 6, 1971 Appi. No.: 169,664

Related US. Application Data Continuation-impart of Ser. No. 863,106, Oct. 2, 1969, abandoned.

U.S. CL 96/ 1 PE; 96/ 1.2 Int. Cl. G03g 13/22; 801k 5/02 Field of Search 96/1, 23, 1.2, 1 R;

Primary Examiner-Charles E. Van Horn Attorney, Agent, or Firm-James .1. Ralabate; David C. Petre; Richard A. Tomlin [57] ABSTRACT A photoelectrophor e'tic imaging system in which selected wavelengths Qf electromagnetic radiation are removed from the imagewise exposing illumination.

2 Claims, 1 Drawing Figure 1 PHOTOELECTROPHORETIC IMAGING PROCESS CROSS REFERENCE TO RELATED APPLICATIONS This application is a continuation-in-part of our copending application Ser. No. 863,l06 filed Oct. 2, I969 in the US. Patent Office, and now abandoned.

BACKGROUND OF THE INVENTION This invention relates in general to imaging methods. More specifically, the invention concerns photoelectrophoretic imaging systems.

There has been recently developed a photoelectrophoretic imaging system capable of producing color images which utilize photoconductive particles. This process is described in detail and claimed in U.S. Pat. Nos. 3,384,565 and 3,384,488 to V. Tulagin and L. Carreira, 3,384,566 to H. Clark and 3,383,993 to 8. Yeh, all issued May 2|, I968. In such an imaging system, various colored light absorbing particles are suspended in a non-conductive liquid carrier. The suspension is placed between electrodes, subjected to a potential difference and exposed to an image. As these steps are completed, selective particle migration takes place in image configuration, providing a visible image at one or both of the electrodes. An essential component of the system is the suspended particles which must be intensely colored and electrically photosensitive and which apparently undergo a net change in charge polarity upon exposure to activating radiation, through interaction with one of the electrodes. The images are produced in color because mixtures of two or more differently colored sets of particles which are each sensitive only to light of a specific wavelength or narrow range of wavelengths are used. Particles used in this system must have both intense pure colors and be highly photosensitive.

In a subtractive color system, the pigment mix includes cyan, magenta and yellow particles. Although the photoelectrophoretic imaging system described above has been found capable of producing high quality monochrome and polychrome images, on occasion these images are found to have a relatively high deposition of pigments in the areas of the final image which are to be clear or white and a lack of pigment particles in image areas which should have a heavy deposit of particles. These variations from a high quality final image are referred to in graphic arts as high Dmin and low Dmax respectively. The color balance of a polychromatic image is also affected by the high Dmin.

lnvariably, in polychrome photoelectrophoretic imaging the spectral sensitivity curves for the various pigments overlap to some degree. Specifically, all yellow and magenta pigments tested have been found to have overlap in the region of from about 0.48 to about 0.52 microns, further the magenta and cyan overlap is in the region of from about 0.58 to 0.6l microns. This overlap causes desaturation of colors in the overlap region because particles are caused to migrate away from the injecting electrode by the action of light of wavelengths to which the particles should not be responsive. Also, pigment response, primarily the cyan pigment, to infrared radiation has been found surprisingly, to cause a high Dmin.

SUMMARY OF THE INVENTION It is, therefore, an object of this invention to provide a method of preparing pigment mixes for electrophoretic imaging processes which overcomes the abovenoted deficiencies.

It is another object of this invention to provide a method for a photoelectrophoretic imaging process which provides relatively improved image background.

lt is another object of this invention to provide a system for photoelectrophoretic imaging which results in images of relatively high quality and correct color balance.

it is still another object of this invention to provide electrophoretic imaging processes capable of producing images having relatively pure, intense colors corresponding to the original.

It is still another object of this invention to provide a process for photoelectrophoretic imaging systems which provides relatively improved blacks.

The above objects and others are accomplished in accordance with this invention by providing an imaging system wherein electrically photosensitive particles dispersed in an insulating liquid are placed in an electrical field and exposed to activating electromagnetic radiation. The electromagnetic radiation is filtered so that radiation in the infrared region, that is, from about 0.7 to 1.0 microns and radiation corresponding to the overlap region between the cyan and the magenta of from about 0.59 to about 0.61 microns is removed. Removing these wavelengths of radiation has been found to provide polychrome images of superior color balance having low Dmin and high Dmax and with improved blacks.

It is speculated that the improvement in image quality when the infrared spectra are removed can be explained as follows. In a preferred embodiment of the imaging process of this invention, the particulate suspension is placed between a transparent conductive or injecting" electrode and a relatively insulating surfaced or "blocking" electrode. Polarities are chosen so that initially particles are drawn toward the injecting electrode. Exposure of the suspension to imagewise radiation is made through the transparent conductive substrate causing particles which are responsive to the radiation to move away from the injecting electrode and adhere to the blocking electrode. The particles remaining on the injecting electrode form an image. Where cyan, yellow and magenta pigments were used responsive mainly to red light, blue light and green light respectively, a full color subtractive image is formed. It is hypothesized that the infrared radiation is able to penetrate the suspension and cause the cyan particles near the blocking electrode which have been found to be responsive to infrared radiation to exchange charge with at least the magenta pigment causing the magenta pigment to move back toward the injecting electrode where it forms a high Dmin and affects the color balance of the final image.

No cyan pigment has been found which does not have the undesirable infrared spectral response. The infrared radiation is also thought to be the source of low Dmax and inferior blacks since it is apparently able to penetrate black or dark areas of a transparency and cause the cyan pigment to leave those areas of the injecting electrode on which it should remain.

Although the above description has been limited to the projection of transparencies, the light reflected from opaque subjects should be similarly filtered where the undesirable infrared and yellow lines are present.

The invention is applicable to monochrome or polychrome imaging.

The transparent conductive electrode may comprise any suitable material. Typical materials include conductively coated glass such as indium oxide coated glass or transparent metallic coatings on transparent plastics. NESA glass, a tin oxide coated glass, is preferred because of its optical clarity.

The blocking electrode may be made of any suitable insulating material. Typical insulating materials include: insulating rubber, baryta paper. polyethylene coated paper, nitro cellulose, polystyrene. polytetrafluoroethylene, polyvinylfluoride, polyethylene terephthalate and mixtures thereof. DuPont Tedlar polyvinyl fluoride film is preferred because it combines high dielectric constant with high dielectric strength and low surface tension which provides excellent cleanability.

The imaging suspension may comprise any suitable electrically photosensitive particles dispersed in a carrier liquid and may be of two or more colors. Typical electrically photosensitive particles and carrier liquids are disclosed in US. Pat. No. 3,384,488 issued May 2l, I968 to V. Tulagin and L. Carreira and US. Pat. No. 3,357,989 issued Dec. I2, 1967 to J. F. Byrne et al., the disclosures of which are incorporated by reference. For monochrome imaging the X-form of phthalocyanine a cyan pigment prepared as in US. Pat. No. 3,357.98) is preferred because of its high sensitivity. For polychrome imaging, a mixture of a cyan pigment, Monolite Fast Blue (3.5. the alpha-form of metal-free phthalocyanine, C. I. No. 74100 available from Arnold Hoffman Co.; a magenta pigment, Bonadur Red B and insolubil ized azo dye available from Collway Colors; and a yellow pigment N2 "-pyridyl-8,13-dioxodinaphtho-(2,1b; 2',3'-d)-furan-6-carboxamide as disclosed in US. Pat. No. 3,447,922 issued June 3, I969 to L. Weinberger, is preferred because of the excellent separation and sensitivity obtained. Sohio Odorless Solvent 3440 is the preferred carrier liquid because of its insulating properties and because it evaporates readily.

BRIEF DESCRIPTION OF THE DRAWING The advantages of this improved method of photoelectrophoretic imaging will become apparent upon consideration of the detailed disclosure of the invention especially when taken in conjunction with the accompanying drawing wherein the FIGURE shows a schematic representation of a simplified exemplary photoelectrophoretic imaging system showing the preferred location of the filters.

Referring now to the FIGURE, there is seen a transparent electrode generally designated 1 which, in this exemplary instance, is made up of a layer of optically transparent glass 2 overcoated with a thin optically transparent layer 3 of tin oxide, commercially available under the name NESA glass. This electrode will hereinafter be referred to as the injecting electrode. Coated on the surface of injecting electrode 1 is a thin layer 4 of finely divided electrically photosensitive particles dispersed in an insulating liquid carrier. The term photosensitive refers to the properties of a particle which once brought into proximity to an electrode will move away from it when exposed to activating electromagnetic radiation. For a detailed theoretical explanation see the above-cited patents. Where this system is used for polychromatic imaging, the suspension will comprise a mixture of two or more differently colored particles; for example, for full natural color imaging cyan, magenta and yellow pigment particles are dispersed in a nonconductive liquid carrier. For monochrome imaging, only, one color is required. Adjacent to liquid suspension 4, is a second electrode generally designated 5, hereinafter referred to as the blocking electrode which is connected to one side of potential source 6 through a switch 7. The opposite side of potential source 6 is connected to the injecting electrode 1 so that when switch 7 is closed an electrical field is applied across imaging suspension 4 between electrodes 1 and 5. An image is projected through electrode l by projecting light from source 8 through lenses l3, transparency l4 and lens 15. Infrared filter 16 is placed between the condensor lenses and a didymium filter I7 is placed between the transparency to be duplicated l4 and lens 15. Transparency 14 may be, for example, a color transparency, such as a Kodachrome" transparency. In this exemplary instance, electrode 5 is made in the form of a roller having a conductive central core 11 and an outer insulating layer which may be, for example, Tedlar. The particle suspension is exposed to the image while a potential is applied by rolling electrode 5 with switch 7 closed across electrode 1. When roller 5 has completed its traverse, a full color positive image corresponding to transparency 14 is found adhering to the surface of electrode 1. This image may usually be improved by repeating the exposure and rol ler traverse at least once. This image may then be transferred by electrostatic transfer or adhesive pick-off or other suitable method.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The following Examples further specifically illustrate the improved photoelectrophoretic imaging system of this invention. Parts and percentages are by weight unless otherwise indicated. The Examples below are intended to illustrate various preferred embodiments of the present invention. All of the following Examples are carried out in apparatus of the general type illustrated in the drawing.

The NESA glass surface is connected to ground, to a potential source, a switch and the conductive center of a roller having a Tedlar surface. The roller is approximately 2%: inches in diameter and is moved across the NESA plate surface at a rate of about 5 centimeters per second. The plate is roughly 3 inches square and is exposed with a light intensity of about 600 foot candles as measured on the uncoated NESA glass surface. Unless otherwise indicated, pigments are suspended in Sohio Odorless Solvent 3440 a mixture of kerosene fractions available from Standard Oil of Ohio, and the magnitude of the applied potential is about 3,000 volts, the roller being made the negative electrode. Exposure is made with a G. E. Tungsten halogen Quartzline lamp CBA v 500 watts through a color Kodachrome" transparency.

EXAMPLE I PRIOR ART An imaging suspension comprising about 3 parts by weight of a cyan pigment, Monolite Fast Blue G8, the alpha form of metal-free phthalocyanine, about 3 parts by weight of a yellow pigment N-2"-pyridyl-8,l 3- dioxodinaphtho-( 2, l-b;2 ,3 '-d )-furan-6-carboxamide and about 5 parts of a magenta pigment Bonadur Red,

B, an insolubilized azo dye C. I. No. I5865 with hydrogen substituted for the sodium substituent dispersed in about 90 parts Sohio Odorless Solvent 3440 is coated onto the NESA glass surface. The suspension is then exposed to the image to be reproduced and the one mil Tedlar covered roller is rolled across the suspension with potential applied. When the roller traverse of the suspension is complete. a full color positive image is found adhering to the NESA glass. A second pass is then made with a second roller to improve the image quality. The image is then transferred to paper. Inspection shows the image to be of fair quality with a magenta Dmin. low Dmax and poor green and yellow rendition.

EXAMPLE II The above experiment is repeated except that a didymium filter is interposed between the transparency and the lens as shown in the drawing. Inspection of the final image shows improvement of green and yellow rendition.

EXAMPLE Ill The experiment of Example I is repeated except that a 90-7 Bausch and Lomb infrared filter is interposed between the condensor lenses as shown in the drawing. Inspection of the final image shows an increase in Dmax and a good Dmin with no magenta in the white areas. Green and yellow color rendition is only fair.

EXAMPLE IV The experiment of Example I is repeated except that the yellow filter of Example II and the infrared filter of Example III are both used. Inspection of the produced image shows excellent color quality. A high Dmax. clean whites and improved color rendition.

EXAMPLE V PRIOR ART EXAM PLE VI The experiment of Example V is repeated except that the yellow and infrared filters are utilized. Inspection of the image produced shows excellent color quality. a high Dmax, clean whites and improved color rendition.

EXAMPLE VII PRIOR ART The experiment of Example I is repeated except that the cyan pigment is replaced by Cyan Blue GTNF the beta form of copper phthalocyanine. C. l. No. 74160 available from Collway Colors. Inspection of the final image shows the image to be of fair quality with a magenta Dmin, low Dmax and poor green and yellow rendition.

EXAMPLE VIII The experiment of Example VII is repeated except that the yellow and infrared filters are placed in the system as indicated by the drawing. Inspection of the final image shows excellent color quality. a high Dmax. clean whites and improved color rendition.

EXAMPLE IX PRIOR ART The experiment of Example I is repeated except that the magenta pigment is replaced with Watchung Red B and the yellow pigment is replaced with Algol Yellow GC, l,2.5.6-di(C,C '-diphenyl )-thiazoleanthraquinone. C. I. No. 67300 available from General Dyestuffs. Inspection of the final image shows the image to be of fair quality with a magenta Dmin. low Dmax and poor green and yellow rendition.

EXAMPLE X The experiment of Example IX is repeated except that the yellow and infrared filters are placed in the system. Inspection of the produced image shows excellent color quality. a high Dmax. clean whites and improved color rendition.

Although specific components and proportions have been described in the above Examples. other suitable materials, as listed above. may be used with similar results. In addition. other materials may be added to the imaging suspension or electrically photosensitive particles to synergize. enhance or otherwise modify their properties. For example. a binder material may be dissolved in the carrier liquid which on evaporation of the liquid will protect the final image.

Other modifications and ramifications of the present invention will occur to those skilled in the art upon a reading of the present disclosure. These are intended to be included within the scope of this invention.

What is claimed is:

I. In the method of photoelectrophoretic imaging wherein a layer of an imaging suspension comprising electrically photosensitive yellow particles. magenta particles and cyan particles dispersed in an insulating carrier liquid and being responsive mainly to blue light. green light and red light. respectively. is provided between a transparent conductive electrode and a second electrode having an insulating surface in contact with said suspension and the suspension is exposed to a pattern of electromagnetic radiation containing wavelengths to which particles of at least two differing colors are sensitive and a potential difference is applied beween said first and said second electrodes until an image is formed. the improvement which comprises removing from said pattern of electromagnetic radiation wavelengths of radiation in the infrared spectral region.

2. The method of claim 1 wherein there is also removed from said pattern of electromagnetic radiation wavelengths in the yellow region of from about 0.58

microns to about 0.6l microns.

* i II 

1. IN THE METHOD OF PHOTOELECTROPHORETIC IMAGING WHEREIN A LAYER OF AN IMAGING SUSPENSION COMPRISING ELECTRICALLY PHOTOSENSITIVE YELLOW PARTICLES, MAGNETA PARTICLES AND CYAN PARTICLES DISPERSED IN AN INSULATING CARRIER LIQUID AND BEING RESPONSIVE MAINLY TO BLUE LIGHT, GREEN LIGHT AND RED LIGHT, RESPECTIVELY IS PROVIDED BETWEEN A TRANSPARENT CONDUCTIVE ELECTRODE AND A SECOND ELECTRODE HAVING AN INSULATING SURFACE IN CONTACT WITH SAID SUSPENSION AND THE SUSPENSION IS EXPOSED TO A PATTERN OF ELECTROMAGNETIC RADIATION CONTAINING WAVELENGTHS TO WHICH PARTICLES OF AT LEAST TWO DIFFERING COLORS ARE SENSITIVE AND A POTENTIAL DIFFERENCE IS APPLIED BETWEEN SAID FIRST AND SAID SECOND ELECTRODES UNTIL AND IMAGE IS FORMED, THE IMPROVEMENT WHICH COMPRISES REMOVING FROM SAID PATTERN OF ELECTROMAGNETIC RADIATION WAVELENGTHS OF RADIATION IN THE INFRARED SPECTRAL REGION.
 2. The method of claim 1 wherein there is also removed from said pattern of electromagnetic radiation wavelengths in the yellow region of from about 0.58 microns to about 0.61 microns. 